1. Field of the Invention
The present invention relates to a carrier for a developer, and a two-component developer containing the same and a toner.
2. Description of the Related Art
An electrophotographic system to which a Carlson process is applied has been widely used in an image forming method using a developer. In an image forming apparatus using the electrophotographic system, an image is formed by performing a charging step, an exposing step, a developing step, a transferring step, a cleaning step, a fixing step, and so forth. In the charging step, a surface of a photoreceptor is uniformly charged in darkness. In the exposing step, an original image is projected onto the charged photoreceptor, resulting in that charges on the surface of the charged photoreceptor exposed to light are removed and an electrostatic latent image is formed thereon correspondingly. In the developing step, a visible image is formed by adhering a toner of a developer onto the electrostatic latent image which is formed on the surface of the photoreceptor. In the transferring step, the toner image is transferred onto a recording medium such as a piece of paper and a sheet by allowing the recording medium to be in contact with the visible image formed on the surface of the photoreceptor and then by performing corona discharge from a side opposite to a contact side of the recording medium with the visible image so as to charge the recording medium with a polarity opposite to that of the toner. In the fixing step, the visible image transferred onto the recording medium is fixed by means of, for example, heating and pressurizing. In the cleaning step, the residual toner remaining on the surface of the photoreceptor without having been transferred onto the recording medium is recovered. By repeating the steps mentioned above, the image forming apparatus using an electrophotographic process forms a desired image on the recording medium.
It has heretofore been known that the developer employed in the image forming apparatus using the electrophotographic system contains the toner produced by a pulverization method, a polymerization method or the like method. The pulverization method is a method in which a thermoplastic resin, a coloring agent, a charge control agent, a wax as an anti-offset agent, and the like agent are melt-kneaded followed by being cooled to be solidified, to thereby prepare a melt-kneaded product, and then the melt-kneaded product thus-prepared is pulverized and classified, to thereby produce the toner. Particularly, in a full color printing, many techniques for making a wax as an anti-offset blend into a toner have been disclosed. Further, the toner can be produced by the polymerization method, for example, a suspension polymerization method or an emulsion polymerization method.
Presently, an image forming apparatus such as a copying machine and a printer, has been downsized and has been made capable of performing high-speed operations. Consequently, in order to obtain a high-quality image for a long period of time, it is required to develop a developer excellent in durability and environmental stability.
In the case of a two-component developer composed of a toner and a carrier, in order for a developer excellent in durability and environmental stability, research and development of carriers such as optimization of core material type, coating resin type and coating resin amount is important for the purpose to stably charge a developer inside an image forming apparatus as well as research and development of toners.
In addition, carriers which have such property as powder characteristics, electrical characteristics, and magnetic characteristics, are required to exhibit developing system-tailored performance. In recent years, in order to improve triboelectric charging property, durability, and environmental stability, a carrier in which a core material is coated with a coating resin layer has come into wider use. By using a carrier in which a core material is coated with a predetermined resin, it is possible to prevent the carrier surface from contamination such as the formation of toner film or the like, thus enabling the achievement of a developer excellent in durability and environmental stability.
Typical related art for developing the carrier mentioned above include, for example, the art disclosed in Japanese Unexamined Patent Publication JP-A 4-177369 (1992). In the color developer described therein, by adding externally titanium oxide or alumina into toner particles, fluidity is imparted to the color developer without deterioration in charging stability. Moreover, durability and environmental stability of the two-component developer are improved by setting the weight of a coating resin (a coating resin layer) to fall in a range of from 0.1% by weight to 5.0% by weight relative to the weight of a carrier core material (a core material), wherein the coating resin contains a nitrogen-containing component for stabilizing chargeability.
Further, in the electrophotographic two-component developer described in Japanese Unexamined Patent Publication JP-A 2003-255591, the coating material resistance to peel-off from the carrier core material is improved by setting the weight of a coating material (a coating resin layer) to be greater than 5.0% by weight relative to the weight of the carrier core material. In addition, environmental property of the two-component developer is improved by adding conductive fine powder such as magnetite (FeO.Fe2O3) to the toner.
Further, it is necessary that the developer has appropriate fluidity and chargeability such that the toner of the developer can be smoothly adhered to an electrostatic latent image formed on a surface of a photoreceptor. The fluidity and the chargeability of the developer vary in accordance with compositions of carrier.
In recent years, the image forming apparatus has been downsized and has been made capable of performing high-speed operations. This leads to that, in forming an image by using the image forming apparatus described above, stress to be given to the developer comes to be larger than that in the conventional image forming apparatus.
In the case of the long-time use of the color developer described in JP-A 4-177369 using the carrier in which the weight of the coating resin falls in a range of from 0.1% by weight to 5.0% by weight relative to the weight of the carrier core material, the coating resin peels off from the carrier core material, thereby causing the surface of the carrier core material to be exposed undesirably. That is to say, in the case of image forming using the downsized image forming apparatus capable of performing high-speed operations, the color developer described in JP-A 4-177369 has insufficient durability incapable of maintaining the carrier characteristics of an initial stage. This undesirably leads to problems with the thus-obtained image, such as a decline in image density.
Meanwhile, in the electrophotographic two-component developer disclosed in JP-A 2003-255591, the weight of the coating material exceeds 5.0% by weight relative to the weight of the carrier core material, resulting in that the surface of the carrier core material is less easily to be exposed even after a long-time use in the downsized image forming apparatus capable of performing high-speed operations. This shows that the developer disclosed in JP-A 2003-255591 is a developer excellent in durability capable of maintaining the carrier characteristics for a long period of time. However, depending on types of coating materials, the coating of the carrier core material with an excessively large amount of the coating material may induce a failure to secure the environmental stability. For this reason, a toner concentration in the developer is unstable in a high-temperature and high-humidity environment, thereby possibly inducing toner scattering, background fogging, or the like. In the art described in JP-A 2003-255591, the problem of deteriorating environmental stability mentioned above is solved by adding externally magnetite to the toner.
In addition, in the case of making a wax as an anti-offset agent blend into a toner, the increase in a blending amount of the wax leads to toner-induced contamination of carrier and deterioration in fluidity of a developer using the toner, thus causing a decline in chargeability, an unevenness of a solid image, or the like problem. This is because the increase in the blending amount of the wax results in difficulty in the dispersion of a part of wax into the toner, even when a kneading operation is applied thereto.
The inventors of the invention have verified that the regulation of compositions and concentration of a toner in which an external additive has been added, enables the achievement of a two-component developer which is excellent in durability and environmental stability even when the weight of a carrier coating resin is 5.0% by weight or more relative to the weight of a core material, and further which is excellent in carrier resistance to toner contamination and makes a developer using the carrier exhibit excellent fluidity, even when the blending amount of wax in the toner is high.